This invention relates to the casting of steel strip.
It is known to continuously cast thin strip in a twin roll caster. In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving casting roll surfaces, and are brought together at the nip between the casting rolls to produce a cast strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of vessels, from which the molten metal flows through a metal delivery nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls so as to restrict the two ends of the casting pool against outflow.
Although twin roll casting with textured casting surfaces has been used with some success for non-ferrous metals which solidify rapidly on cooling (See e.g., U.S. Pat. No. 4,250,950), there have been problems in applying the technique to the casting of ferrous metals. One particular problem has been the achievement of sufficiently rapid and even cooling of metal over the casting surfaces of the rolls. In particular it has proved difficult to obtain sufficiently high cooling rates for solidification onto casting rolls with smooth casting surfaces. It has been proposed to use casting rolls having casting surfaces deliberately textured by regular patterns or random distributions of projections or depressions to control heat transfer and in turn control the heat flux achieved at the casting surfaces during solidification.
For example, our U.S. Pat. No. 5,701,948 discloses a casting roll having textured casting surfaces formed with a series of parallel groove and ridge formations. The depth of the texture from ridge peak to groove root may be in the range 5 to 60 micrometers (μm) and the pitch of the texture should be in the range 100 to 250 μm. The depth of the texture may be in the range 15 to 25 μm and the pitch may be between 150 and 200 μm. In casting thin strip in a twin roll caster, the casting surfaces of the casting rolls with such groove-ridge texture of essentially constant depth and pitch may extend circumferentially around the casting roll. The texture in U.S. Pat. No. 5,701,948 is machined in successive separate annular grooves at regularly spacing along the length of the roll, or in helical grooves machined in the casting surface in the manner of a single start or a multi-start thread. This texture produces enhanced heat flux during metal solidification in order to achieve both high heat flux values and a fine microstructure in the cast steel strip. Although rolls with the texture disclosed in U.S. Pat. No. 5,701,948 have enabled achievement of high solidification rates in the casting of ferrous metal strip, the casting rolls have been found to exhibit a marked sensitivity to casting conditions, which need be closely controlled to avoid two general kinds of strip defects known as “crocodile-skin” and “chatter” defects. It also has been necessary to control sulfur additions to the melt to control crocodile-skin defects in the strip, and to operate the caster within a narrow range of casting speeds to avoid chatter defects.
The crocodile-skin defect occurs when δ and γ iron phases solidify simultaneously in shells on the casting surfaces of the rolls in a twin roll caster, under circumstances in which there are variations in heat flux through the solidifying shells. The δ and γ iron phases have differing hot strength characteristics and the heat flux variations then produce localized distortions in the solidifying shells, which result in the crocodile-skin defects in the surfaces of the resulting strip.
Chatter defects are initiated at the meniscus level of the casting pool where initial metal solidification occurs. One form of chatter defect, called “low speed chatter,” is produced at low casting speeds due to premature freezing of the metal high up on the casting rolls so as to produce a weak shell which subsequently deforms as it is drawn further into the casting pool. The other form of chatter defect, called “high speed chatter,” occurs at higher casting speeds when the shell starts forming further down the casting roll so that there is liquid above the forming shell. This liquid above the forming shell, from the meniscus region, cannot keep up with the moving roll surface, resulting in slippage between the liquid and the roll in the upper part of the casting pool, thus giving rise to high speed chatter defects appearing as transverse deformation bands across the strip.
To address chatter, U.S. Pat. No. 6,942,013 discloses a random texture imparted to a casting roll surface by grit blasting with hard particulate materials such as alumina, silica, or silicon carbide having a particle size of the order of 0.7 to 1.4 mm. For example, a copper roll surface may be grit blasted in this way to impose a desired texture and the textured surface protected with a thin chrome coating of the order of 50 μm thickness. Alternatively, as disclosed in U.S. Pat. No. 7,073,565, the textured casting surfaces of the casting rolls may be formed by a random height distribution of discrete projections typically at least 10 μm in height, where the molten steel used for casting has a manganese content of at least 0.55% by weight and a silicon content in the range of 0.1 to 0.35% by weight. In any case, it is possible to apply a textured surface directly to a nickel substrate with no additional protective coating. A random texture may be achieved by forming a coating by chemical deposition or electrodeposition. Suitable materials include the alloy of nickel, chromium and molybdenum available commercially under the trade name “HASTALLOY C,” and the alloy of nickel, molybdenum and cobalt available commercially under the trade name “T800.”
We have found an improved method of texturing casting rolls providing a substantially regular pattern of raised portions on the casting surface. A method of making textured casting rolls is disclosed comprising the steps of:                (a) forming a plurality of contiguous rows of gravure cells on a casting surface of a casting roll; and        (b) removing portions of the cells to leave raised portions corresponding to raised portions of the gravure cells not removed.        
Though this is not necessary, the gravure cells may be substantially uniform in size. A majority of the raised portions may have a surface area of between 40 and 40,000 μm2, or between 14,000 and 20,000 μm2, or between 900 and 3600 μm2. The contiguous rows of gravure cells formed on the casting roll surface may be between 75 and 250 rows per inch, and may be skewed to the axis of the roll. The rows may be skewed between 5° and 45°.
The steps of forming the gravure cells and of removal of portions of the gravure cells may be done by diamond engraving or by laser, and the removal of the portions of the gravure cells may be done by advancing the starting point in a second pass by a fraction of the width of the gravure cell formed in making the gravure cells. The starting point of the second pass may be advanced between ¼ and ¾ of the width of the gravure cells. Optionally, the step of removing portions of the gravure cells may be done by etching.
The step of forming the gravure cells may be done by diamond engraving, and the step of removal of portions of the gravure cells may be done by etching. The steps of removing the portions of the gravure cells may be done by:                (i) masking the portions not to be etched with a resist, and        (ii) etching the unmasked portions.        
In another embodiment the step of forming the gravure cells may be done by diamond engraving, and the step of removal of portions of the gravure cells may be done by shot blasting.
Where the steps of forming the gravure cells, and the removal of the portions of the gravure cells may also be done by laser. However, in other embodiments where the steps of forming the gravure cells is done by laser, and the step of removal of portions of the gravure cells may be done by forming a resist pattern with a laser and then etching to remove portions of the gravure cells not covered by the resist pattern.
Also disclosed is a textured roll made by the above described method.